1. Field of the Invention
Embodiments of the invention generally relate to methods for increasing the recovery of hydrocarbons from a subterranean reservoir.
2. Description of the Related Art
Oil can generally be separated into classes or grades according to its viscosity and density. Grades of oil that have a high viscosity and density may be more difficult to produce from a reservoir to the surface. In particular, extra heavy oil requires enhanced oil recovery techniques for production. In the following description, the generic term “oil” includes hydrocarbons, such as extra heavy oil, as well as less viscous grades of oil.
A large portion of the world's potential oil reserves is in the form of heavy or extra heavy oil, such as the Orinoco Belt in Venezuela, the oil sands in Canada, and the Ugnu Reservoir in Northern Alaska. Currently, some existing oil reservoirs are exploited using enhanced thermal recovery techniques or solvent-based techniques resulting in a recovery efficiency in the range of 20% to 25%. The most common thermal technique is steam injection by which heat enthalpy from the steam is transferred to the oil by condensation. The heating reduces the viscosity of the oil to allow gravity drainage and collection. Thus, oil recovery is high if the temperature can be maintained near the temperature of the injected steam. Well-known methods such as Cyclic Steam Simulation (“CSS”), Drive Well Injection (“Drive”), and Steam Assisted Gravity Drainage (“SAGD”) may be used to recover oil in the above noted potential reserves.
The CSS method utilizes a single vertical well. Steam is injected into the well from a steam generator at the surface. After allowing the reservoir to soak with the steam for a selected amount of time, the oil is then produced from the same well. When production declines, this process is simply repeated. Further, a pump may be required to pump the heated oil to the surface. If so, the pump is often removed each time the steam is injected, and then replaced after the injection.
The Drive method utilizes a vertical well, known as a drive or injector well, and a laterally spaced nearby well, known as a production well. Steam is continuously injected into the drive well from a steam generator at the surface to heat the oil in the surrounding reservoir. The steam front then drives the heated oil into the production well for production.
The SAGD method utilizes two horizontal wells, one well disposed above and parallel to the other. The upper well is known as the injector well and the lower well is known as the production well. Each well may have a slotted liner. Steam is continuously injected into the upper well to heat the oil in the surrounding reservoir. The steam, with the assistance of gravity, causes the oil to flow and drain into the lower well. The oil is then produced from the lower well to the surface.
These methods have many advantages and disadvantages. As the number of potential oil reservoirs increases and the complexity of the operating conditions of these reservoirs increases, there is a continuous need for more efficient enhanced oil recovery techniques and methods.